MongoDB：如何通过join获取数据

Question

我在mongodb中有以下两个模式

var userSchema = new Schema({
  first_name:  String,
  last_name: String,
  email:   String
});

var messageSchema = new Schema({
  sender: {type: Schema.Types.ObjectId, ref: 'users' },
  receiver: {type: Schema.Types.ObjectId, ref: 'users' },
  message: String,
  sentOn:   Date,
});

var Users = mongoose.model('users', userSchema);
var Messages = mongoose.model('messages', messageSchema);

如何查询来自＆＃39;用户＆＃39;的所有用户？收集和他们的最后一条消息（单个）来自＆＃39;消息＆＃39;集合？